Melting sea ice is affecting the closely-linked Arctic climate in a feedback that will speed up warming there, scientists say.

by
Tim Radford

LONDON,
12 June, 2016 – Scientists have established at least one factor in
the record melting of northern Greenland in 2015. The Arctic itself
played a hand in what happened.

In
a process that engineers call positive feedback, high atmospheric
pressure and clear skies over the Arctic region practically committed
the northwest of Greenland to an episode of melting at record rates.

And
because the Arctic is the fastest-warming region on Earth, and
because atmosphere and ocean influence each other, the steady loss of
sea ice each year has forced a change in wind patterns. This in turn
has played back into the climatic machinery, according to a new study
in Nature Communications.

“How
much and where Greenland melts can change depending on how things
change elsewhere on Earth,” said Marco Tedesco, of Columbia
University’s Lamont-Doherty Earth Observatory, who led the
research.

“If
loss of sea ice is driving changes in the jet stream, the jet stream
is changing Greenland, and this, in turn, has an impact on the Arctic
system as well as the climate. It’s a system, it is strongly
interconnected, and we have to approach it as such.”

Worldwide
impact

Greenland’s
burden of ice is the greatest in the northern hemisphere, and second
only to Antarctica’s. If Greenland’s ice all melted, sea levels
would rise worldwide by seven metres.

So
what scientists call “Arctic amplification” isn’t just a bit of
climate science jargon, it is also a phenomenon that matters
immensely to humans everywhere.

The
process works like this. The Arctic is warming faster as the sea ice
disappears. This is because solar radiation which would have been
reflected by a sheet of ice is being absorbed by blue water, which
speeds up the warming further.

And
since the temperature difference between the Arctic and the tropics
is narrowing, and since it’s the temperature difference that drives
wind and ocean currents, then the jet stream that normally whizzes
around the Arctic circle – thus keeping frozen air in one place and
separating it from the warm breezes of the south – is, the theory
goes, slowing, thus allowing warm moist air to penetrate into the
north. And, it seems, to melt even more of Greenland.

Professor
Tedesco has been observing the intricate interplay of snow, ice, wind
and sunlight in Greenland for years. Last year he and others examined
the record melt of 2012 to piece together the rate of flow of water
to the seas.

“The
conditions we saw in the past aren’t necessarily the conditions of
the future. If humans change the forcing, we are going into uncharted
territory”

Earlier
this year he and others pinpointed a change in albedo – a measure
of the reflectivity of snow on the island – that suggested that
melting might accelerate.

The
latest study doesn’t declare the process of Arctic amplification
guilty as charged: science proceeds cautiously and the researchers
say only that the melting process observed in 2015 is “consistent”
with the hypothesis of Arctic amplification.

In
fact, the jet stream swung north to latitudes never before observed
at that time of year; the winds during July reversed their normal
pattern, and southern Greenland – where melting has been at record
levels for most of the decade – actually saw more snowfall and
lower melting in 2015.

Climate
change is happening, being driven by the human combustion of fossil
fuels at unprecedented rates for more than a century. But how the
climate will change is harder to guess. The suspicion is that – if
the Arctic amplification process works in the way that scientists
think it must do – then climate change in the Arctic can only
accelerate. But right now, all researchers can do is watch, record
and measure.

“The
conditions we saw in the past aren’t necessarily the conditions of
the future,” Professor Tedesco said. “If humans change the
forcing, we are going into uncharted territory.” – Climate News
Network